1. Field of the Invention
This invention relates to operational amplifiers that operate based on relatively high supply voltages applied thereto.
2. Description of the Related Art
FIGS. 4A and 4B show conventional examples of operational amplifiers that operate based on relatively high supply voltages applied thereto. The operational amplifier of FIG. 4A is constituted by a constant current source IS, P-channel Metal-Oxide Semiconductor (PMOS) transistors P1 and P2 for receiving input signals, and N-channel Metal-Oxide Semiconductor (NMOS) transistors N1 and N2 acting as loads of circuitry, wherein a positive supply voltage of 9 V and a negative supply voltage of −9 V are applied to the circuitry. An input signal IP is supplied to a gate of the PMOS transistor P1 via a noninverting input terminal (not shown), while an input signal IN is supplied to a gate of the PMOS transistor P2 via an inverting input terminal (not shown). Compared with the operational amplifier of FIG. 4A, the operational amplifier of FIG. 4B is constituted using resistors r1 and r2, which are substituted for the aforementioned NMOS transistors N1 and N2.
If the input signals IP and IN do not have the same phase, it is possible for the source-drain voltage and the gate-drain voltage of the PMOS transistors P1 and P2 to exceed the prescribed voltage of 7 V; therefore, when transistors of a normal voltage resistant type are used for the PMOS transistors P1 and P2 in the operational amplifiers of FIGS. 4A and 4B, these devices may be destroyed. For this reason, in order to actualize operational amplifiers of a high supply voltage type, it is necessary to use MOS transistors of a high voltage resistant type.
In general, MOS transistors of a high voltage resistant type have a relatively low mutual conductance gm, and they also provide a relatively high gate threshold voltage. This causes a relatively great dispersion in characteristics of MOS transistors. For this reason, when transistors of a high voltage resistant type are used for the PMOS transistors P1 and P2 used in the aforementioned operational amplifiers shown in FIGS. 4A and 4B, it becomes difficult to produce relatively high gains, which are basic characteristics of operational amplifiers. That is, due to unwanted offsets, operational amplifiers must have reduced S/N ratios.